This application is to purchase a gamma-irradiator suitable for the irradiation of small rodents and tissue culture cells, for use by members of the Division of Biology at the California Institute of Technology. Routine access to a gamma-irradiator is crucial for modern cellular immunology and many aspects of cell biology. Irradiation of host animals is usually essential to prepare them for the adoptive transfer of experimental populations of hematopoietic precursors and lymphocytes. Yet no such instrument exists anywhere on the Caltech campus. This has long been an inconvenience, but it has now become an urgent problem since two new immunology laboratories, which depend on access to an irradiator for their research programs, have been recruited to Caltech in the past 1.5 years. At the same time, other Caltech groups have developed a need for this instrumentation, to study DNA-damage cell cycle checkpoints and novel gene transfer protocols. We have facilities into which the irradiator could be safely installed, and we have the support of the Divisional leadership to maintain an irradiator once purchased. We therefore request funds to buy a safe,-general purpose irradiator that is capable of meeting the research needs of a diverse group of users. We plan to buy a J. L. Shepherd Mark 1 Model 68 irradiator with a 2000 Ci 137Cs source. This is a self-contained irradiation facility with three sample turntable positions and an attenuator, enabling it to deliver three different exposure rates, from 540R/min to 130R/min. The sample chamber is large enough to irradiate groups of mice simultaneously. Extra shielding has been requested to keep external radiation levels below 0.5 mR/hr at a distance of 12" from the surface so that the unit can be placed in a conventional room. The manufacturer, J. L. Shepherd, provides full installation and training for multiple operators. This unit would greatly enhance the access of Caltech researchers to an essential tool of modern biology.